SPECIAL TESTS 1 5 



market which is wholly free from diatoms, undissolved agar cells 

 and algal parasites. 



The reason why distilled water should be used in making the 

 solution for centrifugalizing is because ordinary hydrant water 

 may contain diatoms, which might be confusing, especially to a 

 beginner, although the marine diatoms are mostly quite different 

 in form from the fresh water diatoms. With a high-speed centri- 

 fuge less material and less time need be consumed. Also, the 

 more complete the solution the better the results. 



8. Micro -chemical Color Reaction Tests 



There are certain micro-chemical color reactions, other than 

 those already, mentioned, which are of great value in determining 

 the presence of impurities or adulterants in liquids and semi- 

 liquids. The methods as perfected by F. Emich depend upon the 

 use of cotton fibers treated with certain chemicals which convert 

 the metallic compounds into the sulphides. The prepared threads 

 can be readily transferred to the several solutions used and the 

 color and precipitation effects can be observed under the micro- 

 scope. The following are the more important reagents and 

 reactions : 



1. Cotton Threads for Metal Tests. Dip absorbent cotton threads alternately 

 into 15 per cent, solutions of sodium sulphide and zinc sulphate, pressing between 

 blotting paper, and air-dry each time. 



The threads thus prepared should assume a deep black color with a i per cent, 

 solution of silver nitrate. They may be kept for a long time and are used to demon- 

 strate the presence of As, Sb, Au, Pt, Cu, Hg, Pb and Bi, in various chemical 

 compounds. 



2. Ammonium Sulphide Vapor Test. Place a few fibers of absorbent cotton into 

 a drop of the suspected solution and allow the moisture to evaporate. Suspending 

 the threads in the vapor of ammonium sulphide will indicate the presence of Cd, 

 Hg, Ag, Fe, Co and Ni (dark to black coloration). 



The prepared threads are used in the following tests: 



a. Arsenical Test. Dip a sodium sulphide thread into the suspected solution 

 and allow to dry. In the presence of 0.008 per cent, arsenic there is a distinct yel- 

 lowish coloration, due to the sulphide of arsenic formed in and upon the threads. 

 The arsenical threads will also show the characteristic reactions with hydrochloric 



